Electrokinetic phenomena include electromigration, electroosmosis, and electrophoresis. Electroosmosis is defined as the mass flux of a fluid containing ions through a stationary porous medium caused by the application of an electrical potential. The fluid moves through the voids in the porous medium (e.g. soil) called pores. Each pore has a thin layer of charged fluid next to the pore wall having a typical thickness of between about 1 and about 10 nanometers. The thin layer of charged fluid next to the pore wall is present to neutralize the charge on the surface of the soil particle that forms the pore wall. Fluid movement occurs in soil pores because of the charge interaction between the bulk of the liquid in the pore and the thin layer of charged fluid next to the pore wall. Under the influence of a DC electric field, the thin layer of charged fluid moves in a direction parallel to the electric field. Large amounts of liquid may be transported along with the thin layer of charged fluid as well as contaminants or other species contained within the liquid.
Electromigration is defined as the mass flux of a charged ionic or polar species within a liquid or solution from one electrode to another electrode. Electromigration and electroosmosis may occur simultaneously and are the dominant mechanisms through which conventional electrokinetic transport processes occur.
Electroosmosis has been used as a method for dewatering soils and sludges. The fundamentals of this method were established through the work of Cassagrande and Grey. During the 1970's, electrokinetic metal recovery was used as a method for mining metals, such as copper. These processes involved insertion of electrodes into the ground. The electrodes are enclosed within porous enclosures or wells. These enclosures are filled with an electrolyte, typically an acid.
One recent application in which electrokinetic transport of materials has found practical use is the electrokinetic remediation of contaminants in soil. Electrokinetic remediation, frequently referred to as either electrokinetic soil processing, electromigration, electrochemical decontamination or electroreclamation, uses electrical currents applied across electrode pairs placed in the ground to extract radionuclides, heavy metals, certain organic compounds, or mixed inorganic species and organic wastes from soils and slurries. The contaminants in a liquid phase in the soil are moved under the action of the electrical field to wells where they are then pumped out.
During electrokinetic processing, water in the immediate vicinity of the electrodes is electrolyzed to produce H+ ions at the anode and OH- ions at the cathode, causing the pH of the soil to change, according to the following equations.
Anode Reaction EQU 2H.sub.2 O.fwdarw.O.sub.2 +4e.sup.- +4H.sup.+ Equation (1)
Cathode Reaction EQU 2H.sub.2 O+2e.sup.-.fwdarw.H.sub.2 +2OH.sup.- Equation (2)
If the ions produced are not removed or neutralized, these reactions lower the pH at the anode and raise the pH at the cathode. Protons formed at the anode migrate towards the cathode and can aid contaminant removal by increasing metal extraction. In contrast, the hydroxyl ions formed at the cathode do not migrate as efficiently as protons and can increase the soil pH in the cathode region, as high as a pH of 12, and cause deposition of insoluble species, thereby forming regions of high electrical resistivity. These pH changes can have a significant effect on the soil zeta potential as well as solubility, ionic state and charge, and the level or adsorption of the contaminants. It is, therefore, desirable to monitor and control the pH of the fluids in the vicinity of the electrodes as well as the fluid transported from the anode to the cathode.
In areas with highly porous mediums, such as sand, it is difficult to stop the downward drainage of fluids including contaminants. It is therefore desirable to control the flow of fluids through highly porous mediums and overcome the gravity induced downward drainage of the fluids.
The voltage drop across the well wall and the soil effects the rate of electroosmotic flow depending on the type of soil being remediated. It would be useful if the voltage drop across the soil and the well wall could be controlled so as to maximize electroosmotic flow through a porous medium.
There is a central dilemma regarding the make up of well walls for the electrodes. For efficient electrokinetic processing, the well walls must be highly permeable to fluids and to ions. However, to prevent downward drainage, the well must be highly impermeable to fluids. Ideally, the well wall should provide "fluid rectification", meaning the well wall does not prevent electrokinetic processes (either electrokinetically driven ion migration or the flow of fluid occurring by electroosmosis) and inhibits the movement of fluid by other means.
Electroosmosis is a very important phenomenon that can be harnessed for fluid removal from soil or for the introduction of fluids into the soil. However, there are difficulties with electroosmosis, in particular, certain soil conditions do not support electroosmosis, such as soil with relatively high hydraulic permeability (i.e., relatively loosely packed sandy soils). Electroosmosis is most effective in fine-grained soils with pore sizes of about a micrometer or smaller, such as clayey or silty soils. With sandy soils, gravitational flow and downward drainage are usually the dominant fluid flow processes. It is therefore desirable in certain circumstances to design electrode well walls that inhibit downward drainage and promote electroosmosis in loosely packed soils.